In semiconductor processing a low pressure (vacuum) chamber is often used to process substrates. Process gas is introduced to process the substrate and the depleted process gas is evacuated from the processing chamber by a vacuum system.
The vacuum system includes rough vacuum (.about.1 torr) piping system connected to many processing chambers. Each processing chamber such as illustrated at 10 in FIG. 1 is separated from a normal rough vacuum system piping 14 (connected to the rough vacuum system) by a throttling valve 11, a gate valve 12, a turbo vacuum pump 13 (capable of creating a fine vacuum of .about.10.sup.-11 torr--however, typical maximum vacuum in the process chamber is .about.10.sup.-6 torr) and an isolation valve 20. The localized vacuum pump 13 often assists the rough vacuum piping system 14 in providing vacuum in the processing chamber. The pump is typically a turbo pump having a magnetically levitated turbo rotor (e.g., Leybold 340 MCT). When the chamber 10 is at a pressure higher than that provided by the rough vacuum system 14, and the throttling and gate valves 11, 12 are open, the process chamber 10 is isolated from the rough vacuum system only by the seal of the isolation valve 20 (e.g., HPS Division of MKS Instruments, Inc. Model 172.0040K). Even though the turbo vacuum pump 13 may be spinning, since its exhaust is sealed, it has little effect on the pressure in the processing chamber 10.
To bring the process chamber to a vacuum, the electric solenoid vent valve 29 is energized, thereby pressurizing an air cylinder 21 of the isolation valve 20 through a quick exhaust valve 19 (e.g., Clippard Instrument Laboratory, Model Minimatic MEV-2). The air pressure pressurizes a piston 27. The piston's movement is opposed by a spring 24. When sufficient pressure and thereby resulting force is introduced, the piston will move with the valve stem to lift the stem from its seat 26, thereby creating an opening through which gas will surge from the high pressure processing chamber 10 to the rough vacuum system connected to the rough vacuum piping 14. The rapid reduction in pressure on the downstream side of the turbo pump 13 causes there to be at least initially a high differential pressure across the turbo pump rotor. The force associated with the differential pressure causes the rotor to be suddenly pushed from its electro-magnetically held levitating position. If the differential pressure across the rotor is sufficiently high (&gt;200mtorr), the spinning rotor may contact its touchdown bearing and may damage it such that the rotor can no longer be used. When the turbo rotor contacts the touchdown bearing brinelling, galling, and premature wear of the touchdown bearing occurs. Such contacts make turbo pump failure imminent. Extensive and time-consuming repairs must then be undertaken as a minimum. In certain instances the turbo pump is not repairable (when the turbo blades touch the stator blades, resulting in severe damage to the blades--known as "helicoptoring").
To prevent rotor bearing contacts the isolation valve must be opened slowly. To safeguard against accidents and disruptions of the rough vacuum piping system, causing backstreaming into the process chamber, the rapid valve closing characteristics (of preferably less than 500 milliseconds) must be maintained.
While orifices are well known as flow and pressure retardants, their use in hindering flow into and out of pneumatic valves is not effective when the orifices plug. When the flow rate to be controlled is quite small, the orifice size must be tiny. When tiny orifices are used, dirt particles can and often do plug the orifice, rendering the flow control system inoperative. The tiny orifices available usually require piping disassembly for access and therefore are not easily maintained. No production-worthy technique exists to warn the operator if plugging of an orifice is imminent.
Further, pneumatic control circuits often have several solenoid valves, one for gas inlet, another for gas exhaust that need to work cooperatively in order for the system to function properly. In these cases both solenoids must function perfectly in coordination for the system to operate acceptably. The problem of orifice sizing and plugging and the need for coordination between several electric devices is a hindrance to field installation or retrofit of new and preexisting devices.
Also, the internal mechanical friction characteristics vary from isolation valve to isolation valve, requiring that the orifice size be tailored to match performance with the valve internal friction. During manufacturing the piston rod seal and piston seal are lubricated. Over time the lubricant is depleted. Once the lubricant is depleted destructive wear begins. The wear creates gaps through which the pressurized air leaks. When the cylinder air leaks equal the flow through the transmission line flow restriction there is no cylinder movement. To overcome these drawbacks, a variable size inlet orifice would be required, the sizing of which would be determined by the changing valve friction and leakage rates.
For example, in one valve-valve air cylinder combination a properly sized orifice is 0.010" (0.25 mm) (the next smaller size (i.e., 0.007" (0.18 mm)) is insufficient to provide air pressure to the valve to operate it properly or at all (the air leaking around the piston flows more quickly than the air through the small orifice). In another valve-valve air cylinder combination a 0.010" orifice might be too small, requiring a larger orifice (0.020" to 0.030" (1.50 mm to 0.75 mm)) to operate properly. Using a 0.020" to 0.030" (1.50 mm to 0.75 mm) orifice in the first case would cause the turbo pump rotor to crash. Therefore orifice sizing is not universal and a variable orifice or other production-worthy methods must be used.
The use of an adjustable pressure regulator is also impractical, because the setting would have to be adjusted only after listening for and hearing the turbo rotor crashing onto its touchdown bearing.